Substrates such as, for example, glass panels used in applications such as television and computer displays may be fabricated using sequential steps including physical vapor deposition (PVD), chemical vapor deposition (CVD), etching, and annealing to produce the desired device. These steps may be carried out using a variety of processing systems having multiple chambers. One such system is known as a “cluster tool”. A cluster tool generally includes a central wafer handling module or transfer chamber and a number of peripheral chambers including a loadlock chamber through which workpieces are introduced into and removed from the system and a plurality of other chambers for carrying out processing steps such as heating, etching, and deposition. The cluster tool also generally includes a robot for transferring workpieces between the chambers.
The processing of large glass substrates used for displays is in some ways similar to the processing of other types of substrates such as semiconductor wafers. Such glass substrates, however, are often larger than typical silicon wafers. For example, glass substrates may have dimensions of 550 mm by 650 mm, with trends towards even larger sizes such as 650 mm by 830 mm and larger, to permit the formation of larger displays. The use of large glass substrates introduces complexities into processing that may not be present when processing other types of substrates. For example, in addition to their size, glass substrates used for displays are typically rectangular in shape. The large size and shape of glass substrates can make them difficult to transfer from position to position within a processing system when compared with smaller, circular-shaped substrates. As a result, systems for processing glass substrates generally require larger chambers, apertures, and transfer mechanisms. In addition, it is known to utilize large cassettes holding approximately 12 substrates within the loadlock in order to supply a large number of substrates to the processing chambers for batch processing operations. The need for larger chamber sizes to handle large substrates, as well as the use of large substrate cassettes in the loadlock, also leads to a requirement for larger and more powerful vacuum pumps, power supplies, control mechanisms and the like and a corresponding increase in system cost.
In addition, glass substrates often have different thermal properties than silicon substrates. In particular, glass generally has a relatively low thermal conductivity, which can make it more difficult to uniformly heat and cool the substrate. Temperature gradients may occur across the glass substrate, which can lead to undesirable stresses in the substrate upon cooling. The heat loss near the substrate edges tends to be greater than near the center. Temperature gradients during processing can also result in the components formed on the substrate surface having non-uniform electrical (and structural) characteristics. As a result, to maintain adequate temperature control, heating and cooling operations are often performed relatively slowly. As the system components become larger in size, these steps may be slowed even more due to the longer time it generally takes to heat and cool large components in a large volume chamber. These slow operations tend to lower the system throughput.